Ultra high frequency

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Ultra high frequency
Frequency range 0.3 to 3 GHz
ITU Radio Band Numbers

1 2 3 4 5 6 7 8 9 10 11
ITU Radio Band Symbols

ELF SLF ULF VLF LF MF HF VHF UHF SHF EHF
NATO Radio bands

A B C D E F G H I J K L M
IEEE Radar bands

HF VHF UHF L S C X Ku K Ka Q V W
v d e

Ultra high frequency (UHF) designates a range of electromagnetic waves with frequencies between 300 MHz and 3 GHz (3,000 MHz), also known as the decimetre band or decimetre wave as the wavelengths range from one to ten decimetres (10 cm to 1 metre). Radio waves with frequencies above the UHF band fall into the SHF (super high frequency) and EHF (extremely high frequency) bands, all of which fall into the microwave frequency range. Lower frequency signals fall into the VHF (very high frequency) or lower bands. See Electromagnetic spectrum and Radio spectrum for a full listing of frequency bands.

Contents

[edit] Characteristics, advantages, and disadvantages

Main article: radio propagation

The point to point transmission and reception of TV and radio signals is affected by many variables. Atmospheric moisture, solar wind, physical obstructions such as mountains and buildings, and time of day will all have an effect on the signal transmission and degradation of signal reception. All radio waves are partially absorbed by atmospheric moisture. Atmospheric absorption reduces, or attenuates, the strength of radio signals over long distances. The effects of attenuation degradation increases when switching from VHF TV signals to UHF TV signals. UHF TV signals are generally more degraded by moisture than lower bands such as VHF TV signals. The layer of the Earth's atmosphere called the ionosphere is filled with charged particles that can reflect some radio waves. Amateur radio enthusiasts primarily use this quality of the ionosphere to help propagate low frequency VHF signals around the world: the waves are trapped, bouncing around in the upper layers of the ionosphere until they are refracted down at another point on the earth. This is called skywave transmission. UHF TV signals do not have the benefit of being carried along the ionosphere but can be reflected off of the charged particles down at another point on earth in order to reach farther than the typical line-of-sight transmission distances, this is called the skip distance. UHF transmission and reception will be enhanced or degraded by tropospheric ducting as the atmosphere warms and cools throughout the day. The main advantage of UHF transmission is the physically short wave that is produced by the high frequency. The size of transmission and reception antennas, is related to the size of the radio wave. The UHF antenna is stubby and short. Smaller and less conspicuous antennas can be used with higher frequency bands. The major disadvantage of UHF is its limited broadcast range and reception, often referred to as line-of-sight between the TV station's transmission antenna and customer's reception antenna, as opposed to VHF's very long broadcast range and reception which is less restricted by line-of-sight.

UHF is widely used in two-way radio systems and cordless telephones whose transmission and reception antennas are closely spaced. UHF signals travel over line-of-sight distances. Transmissions generated by two-way radios and cordless telephones do not travel far enough to interfere with local transmissions. A number of public safety and business communications are handled on UHF. Civilian applications such as GMRS, PMR446, UHF CB, and 802.11b ("WiFi") are popular uses of UHF frequencies. A repeater is used to propagate UHF signals when a distance that is greater than the line-of-sight is required.

[edit] History

[edit] Australia

In Australia, UHF was first anticipated in the mid 1970s with TV channels 27 - 69. The first UHF TV broadcasts in Australia were operated by Special Broadcasting Service (SBS) on channel 28 in Sydney and Melbourne starting in 1980, and translator stations for the Australian Broadcasting Corporation (ABC). The UHF band is now used extensively as ABC, SBS, commercial and public-access television services have expanded, particularly through regional areas.

Australia also provides the UHF CB service for general-purpose two-way communications.

[edit] Canada

The first Canadian television network was publicly-owned Radio-Canada, the Canadian Broadcasting Corporation. Its stations, as well as that of the first private networks (CTV and TVA, created in 1961), are primarily VHF. More recent third-network operators initially signing-on in the 1970s or 1980s were often relegated to UHF, or (if they were to attempt to deploy on VHF) to reduced power or stations in outlying areas. Canada's VHF spectrum was already crowded with both domestic broadcasts and numerous foreign border stations.

The use of UHF to provide programming which otherwise would not be available, such as province-wide educational services (Knowledge Channel, TVOntario, Télé-Québec), French language programming (outside Québec) and ethnic/multilingual television, has therefore become common. Third networks such as Quatre-Saisons or Global often will rely heavily on UHF stations as repeaters or as a local presence in large cities where VHF spectrum is largely already full. The handful of digital terrestrial television stations currently on-air in Canada as of 2008 are also all UHF broadcasts, although some digital broadcasts will return to VHF channels vacated after the digital transition is completed in August 2011.[1]

Digital Audio Broadcasting, deployed on a very limited scale in Canada in 2005, uses UHF frequencies in the L band from 1452-1492 MHz. There are currently no VHF Band III digital radio stations in Canada as, unlike in much of Europe, these frequencies are among the most popular for use by television stations.[2]

[edit] Ireland

In the Republic of Ireland, UHF was introduced in 1978 to augment the existing RTÉ One VHF 625-line transmissions and to provide extra frequencies for the new RTÉ Two channel. The first UHF transmitter site was Cairn Hill in Co. Longford, followed by Three Rock Mountain in South Co. Dublin. These sites were followed by Clermont Carn in Co. Louth and Holywell Hill in Co. Donegal in 1981. Elsewhere in Ireland, both the RTÉ channels are available on VHF. Since then RTÉ have migrated nearly all their low-power relay sites to UHF. TV3 and TG4 are transmitted entirely in UHF only. When Digital Terrestrial TV is introduced, it is intended to broadcast this on UHF only initially, although VHF allocations exist. VHF TV is likely to cease whenever the existing analogue broadcasts are switched off. The UHF band is also used in parts of Ireland for Television deflector systems bringing British television signals to towns and rural areas which cannot receive these signals directly

[edit] Japan

In Japan, an Independent UHF Station (ja:全国独立UHF放送協議会 Zenkoku Dokuritsu Yū-eichi-efu Hōsō Kyōgi-kai?, literally National Independent UHF Broadcasting Forum) is one of a loosely-knit group of free commercial terrestrial television stations which is not a member of the major national networks keyed in Tokyo and Osaka.

Japan's original broadcasters were VHF. Although some experimental broadcasts were made as early as 1939, NHK (founded in 1926 as a radio network modeled on the BBC) began regular VHF television broadcasting in 1953. Its two terrestrial television services (NHK General TV and NHK Educational TV) appear on VHF 1 and 3 respectively in the Tokyo region. Privately-owned Japanese VHF TV stations were most often built by large national newspapers with Tokyo stations exerting a large degree of control over national programming.

The independent stations broadcast in analogue UHF, unlike major networks which were historically primarily broadcast in analogue VHF. The loose coalition of UHF independents is operated mostly by local governments or metropolitan newspapers with less outside control. Compared with major network stations, Japan's UHF independents have more restrictive programming acquisition budgets and lower average ratings; they are also more likely to broadcast single episode or short-series UHF anime (many of which serve to promote DVD's or other product tie-ins) and brokered programming such as religion and infomercials.

Japanese terrestrial television is in the process of switching entirely to digital UHF, with all analogue television (both VHF and UHF) planned to shut down in 2011.

[edit] Malaysia

UHF broadcasting was used outside Kuala Lumpur and the Klang Valley by private TV station TV3 in the late 80s, with the government stations only transmitting in VHF (Bands 1 and 3) and the 450 MHz range being occupied by the ATUR cellular phone service operated by Telekom Malaysia. The ATUR service ceased operation in the late 90s, freeing up the frequency for other uses. UHF was not commonly used in the Klang Valley until 1994 (despite TV3's signal also being available over UHF Channel 29, as TV3 transmitted over VHF Channel 12 in the Klang Valley). 1994 saw the introduction of the channel MetroVision (which ceased transmission in 1999, got bought over by TV3's parent company - System Televisyen Malaysia Berhad - and relaunched as 8TV in 2004). This was followed by Ntv7 in 1998 (also acquired by TV3's parent company in 2005) and recently Channel 9 (which started in 2003, ceased transmission in 2005, was also acquired by TV3's parent company shortly after, and came back as TV9 in early 2006). At current count, there are 4 distinct UHF signals receivable by an analog TV set in the Klang Valley: Channel 25 (8TV), Channel 29 (TV3 UHF transmission), Channel 37 (NTV7) and Channel 39 (TV9). Channel 39 is usually allocated for VCRs, decoder units (i.e. the ASTRO and MiTV set top boxes) and other devices that have an RF signal generator (i.e. game consoles).

[edit] United Kingdom

In the UK, UHF television began in 1964 following a plan by the General Post Office to allocate sets of frequencies for 625-lined television to regions across the country, so as to accommodate four national networks with regional variations (the VHF allocations allowed for only two such networks using 405 lines). The UK UHF channels would range from 21 to 68 (later extended to 69) and regional allocations were generally grouped close together to allow for the use of aerials designed to receive a specific sub-band with greater efficiency than wider-band aerials could. Aerial manufacturers would therefore divide the band into over-lapping groups; A (channels 21-34), B (39-53), C/D (48-68) and E (39-68). The first service to use UHF was BBC2 in 1964 followed by BBC1 and ITV (already broadcast on VHF) in 1969 and Channel 4/S4C in 1982. PAL colour was introduced on UHF only in 1967 (for BBC2) and 1969 (for BBC1 & ITV).

As a consequence of achieving maximum national coverage, signals from one region would typically over-lap with that of another, which was accommodated for by allocating a different set of channels in each adjacent area, often resulting in greater choice for viewers when a network in one region aired different programmes to the neighbouring region.

Initial uptake of UHF television was very slow: Differing propagation characteristics between VHF and UHF meant new additional transmitters needed to be built, often at different locations to the then-established VHF sites, and generally with a larger number of relay stations to fill the greater number of gaps in coverage that came with the new band. This led to poor picture quality in bad coverage areas, and many years before the service achieved full national coverage. In addition to this, the only exclusively UHF service, BBC2, would run for only a few hours a day and run alternative programming for minority audiences in contrast to the more populist schedules of BBC1 and ITV. However the 1970s saw a large increase in UHF TV viewing while VHF took a significant decline: The appeal of colour, which was never introduced to VHF (despite preliminary plans to do so in the late 1950s and early 1960s) and the fall in television prices saw most households use a UHF set by the end of that decade. With the second and last VHF television service having launched in 1955, VHF TV was finally decommissioned for good in 1985 with no plans for it to return to use.

The launch of Channel 5 in 1997 added a fifth national television network to UHF, requiring deviation from the original frequency allocation plan of the early 1960s and the allocation of UHF frequencies previously not used for television (such as UK Channels 35 and 37, previously reserved for RF modulators in devices such as domestic videocassette recorders, requiring an expensive VCR re-tuning programme funded by the new network). A lack of capacity within the band to accommodate a fifth service with the complex over-lapping led to the fifth and final network having a significantly reduced national coverage compared to the other networks, with reduced picture quality in many areas and the use of wide-band aerials often required.

The launch of digital terrestrial television in 1998 saw the continued use of UHF for television, with six multiplexes allocated for the service, all within the UHF band. However analogue transmissions have been planned to cease completely by 2012 after which time it is uncertain as to whether the vacated capacity will be used for additional digital television services or put into alternative use, such as mobile telecommunications or internet services.

[edit] United States

[edit] Television

On December 29, 1949, KC2XAK of Bridgeport, Connecticut, became the first UHF television station to operate on a regular daily schedule. The first commercially-licensed UHF television station on the air was KPTV, Channel 27, in Portland, Oregon, on September 18, 1952. This TV station used much of the equipment, including the transmitter, from KC2XAK.

American television broadcasting, which began experimentally in the 1930s with some regular commercial broadcasting in just a few cities (such as New York and Chicago) in 1941, was originally allocated (by the Federal Communication Commission - the FCC) broadcasting channels solely in the VHF (Very High Frequency) band. All VHF TV channels except channel 1 through 13 had been removed from the FCC allocation list during World War II and those frequencies re-allocated for military use, leaving thirteen channels as of May 1945.[3] While efforts at TV broadcasting on any channel were drastically curtailed for the duration of WW II, largely due to lack of available receivers, the post-war era would bring rapid expansion in the nascent broadcast television industry.

After VHF Channel 1 was re-allocated to land-mobile radio systems in 1948 due to radio-interference problems, a mere one dozen TV channels remained. These were found to be not enough to serve the needs of television broadcasting as it grew nationwide during the latter 1940s and the 1950s. For example, these cities were never able to be allocated any VHF-TV stations at all, due to technical reasons found by the FCC: Fort Wayne, Indiana, Lexington, Kentucky, Huntsville, Alabama, and Fresno, California. In addition, scores more cities were able to receive only one VHF broadcast station, for example High Point, North Carolina, Montgomery, Alabama, Wilmington, Delaware, Bakersfield, California, and Santa Barbara, California. Also, the entire state of New Jersey would receive only one VHF broadcast station of its own (which was to ultimately become WNET 13 Newark), leaving much of the state to be served from New York City or Philadelphia, and Delaware has had only one VHF station. Clearly, there was a problem with an insufficient number of TV channels being available to cover all of the United States.

With a grand total of 106 VHF stations broadcasting by the end of the 1940s in the U.S., problems with interference between stations due to some overcrowding of stations were already becoming apparent in the densely-populated areas, such as the eastern mid-Atlantic states (New York, Pennsylvania, New Jersey, Delaware, Maryland, and Connecticut) and Southern California. In 1949, the Federal Communications Commission stopped accepting applications for licensing new stations (a freeze that lasted until 1952) in order to address questions such as the allocation of additional channel frequencies, and also the selection of an electronic system for color television.

Allocating more of the VHF band (30 to 300 megahertz - MHz ) by moving existing radio communication users off this band seemed to be impossible. For example, FM radio broadcasting had already suffered a huge setback after a forced move from its original 42 to 50 MHz allocated band to the current 88 to 108 MHz band in 1946[4] rendered all existing FM transmitters and receivers obsolete. Furthermore, several other important radio communications services use the VHF band. For example, in aeronautical radio use, a so-called "UHF radio" system for voice communications actually falls in VHF spectrum as all of its frequencies are below 300 MHz. The aeronautical radio VHF radio system, located above 108 MHz, is among the frequencies fall into the wide band that is in between Channel 6 and Channel 7 of VHF broadcast TV. Police and fire department radios, land-mobile users and two-metre amateur radio operators also occupy VHF Band II, along with the entire FM broadcast band. It was impractical and uneconomic to require these well-established users to move to other frequencies, such as to the genuine UHF band (300 MHz to 3.0 GHz).

The U.S. Army and Navy (and the new Air Force - established 1947) did not need to keep their huge wartime UHF spectrum allocation simply because they had never used most of it. That allocation had been done hastily in 1942 in the face of the emergency of a huge war of unknown duration - and with the presence of very new and poorly-understood electronic technologies like radar. In 1942, nobody knew how much bandwidth that the Army and the Navy might need for radar and for radio communications, so the Federal Government took a wise expedient: it allocated a huge amount of radio spectrum to the uniformed services for the time being, in case the service might need it. Then, it could make adjustments later.

After the War ended, and after the growth of civilian TV broadcasting in the years after the war, by 1950 expansion of TV channels into the UHF band of frequencies became inevitable. However, lots of UHF TV technology remained unproven at that time, though plenty had been learned about UHF electronics during the war, especially in the development and improvement of radar. (There are significant advantages to using shorter wavelengths, hence higher frequencies, for radars.) Also, the question of which owners should retain the more-valuable (at that time) VHF TV channels remained hotly contested between several different competing interests.

To incumbent corporations, such as the Radio Corporation of America and its National Broadcasting Company subsidiary, UHF-TV and FM radio represented disruptive technologies - competition to their existing and long-established manufacturing and broadcast interests in VHF-TV and AM radio. To the second-ranked radio network the Columbia Broadcasting System, the allocation of the UHF spectrum to permit two channels of color TV or of some form of high-definition television[citation needed][dubious – discuss] appeared to be more important than the use of the channels to provide more channels to individual cities. To newer entrants into TV broadcasting such as the DuMont Laboratories company and its fourth-ranked DuMont Television Network, however, the need for additional TV channels in major markets was urgent.[5] For proponents of educational TV broadcasting, the difficulties in competing with commercial broadcasters for the increasingly-scarce VHF channels becoming a key problem.[6]

Any attempt to pursue the objective of broadcast localism on the VHF-TV channels threatened in many regions to push the third-network TV companies such as the American Broadcasting Company onto stations in outlying communities, if they could be accommodated on VHF channels at all.

A key question in the FCC's allocation of TV channels was hence that of intermixture.[clarification needed] To allocate four to as many as seven VHF channels to each of the largest cities would mean forcing the smaller, intervening cities completely onto the UHF channels, while an allocation scheme that sought to assign one or two VHF channels in each smaller city would force VHF and UHF stations to compete in most markets. (Some may find it hard to believe, but the large metropolitan areas of New York City, Washington-Baltimore, Los Angeles, and San Francisco received seven VHF stations apiece, and Chicago was allocated five, with the other two possible ones going to Milwaukee and Rockford, Illinois.)

Primary Network Affiliates May 1954.png

Hopes that UHF-TV would allow dozens of television stations in every media market were thwarted not only by poor image frequency rejection in superheterodyne receivers with the standard intermediate frequency of 45.75 MHz, but also by very poor adjacent-channel rejection and channel selectivity by early tuner designs and manufactures. UHF-TV stations in the same immediate area were usually assigned by the FCC a minimum of six channels apart due to inadequate TV receiver manufacture. Technical problems with the design of vacuum tubes for operation at high UHF frequencies were beginning to be addressed as late as 1954.[7] These shortcomings led to "UHF taboos", which in effect limited each metropolitan area to only moderately more UHF stations than VHF ones, despite the theoretically much higher number of channels.[8]

While the more-established broadcasters were operating profitably on VHF channels as affiliates of the largest TV networks (at the time, NBC and CBS), most of the original UHF local stations of the 1950s soon went bankrupt, limited by the range their signals could supposedly travel, the lack of UHF tuners in most TV sets, and difficulties in finding advertisers willing to spend money on them. TV network affiliations were also difficult to get in many locations; the UHF stations with major-network affiliation would often lose these affiliations in favour of any viable new VHF TV station which entered the same market. Of the 82 new UHF-TV stations in the United States broadcasting as of June 1954, only 24 of them remained on the air a year later.[9] The fraction of new TV receivers that were factory-equipped with all-channel tuners dropped from 35% in early 1953 to 9.0% by 1958, a drop that was only partially compensated for by field upgrades or the availability of UHF converters for separate purchase.

The majority of the 165 UHF stations to begin telecasting between 1952 and 1959 did not survive. Under the All-Channel Receiver Act, FCC regulations by 1965 would ensure that all new TV sets sold in the U.S. had built-in UHF tuners that could receive channels 14-83. In spite of this, by 1971, there were just more than 170 full-service UHF broadcast stations nationwide.[10]

[edit] Independent and educational stations

In the United States, the UHF stations gained a reputation for being locally owned, less polished and professional, not as popular, and having weaker signal propagation than their VHF channel counterparts.

While UHF-TV has been available to American TV broadcasters since 1952, affiliates of the four major American TV networks (NBC, CBS, ABC, and DuMont) continued to transmit their programs primarily on VHF channels wherever they were available. With the availability of the twelve VHF television channels limited by FCC spacing rules to avoid co-channel and adjacent channel interference between TV stations in the same or nearby cities, all available VHF-TV allocations were already in use in most large TV markets by the mid-1950s.

To be more specific, two TV stations on the same channel needed to be about 160 or more miles apart, and two TV stations on adjacent channels needed to be about 60 or more miles apart. Exceptions to this rule occurred with VHF channels 4 and 5, and VHF channels 6 and 7, because there are additional "guard bands" between these two pairs that are allocated to other uses. Thus, the pair channel 4 and 5 was found in New York City, Washington, D.C., St. Louis, Los Angeles, San Francisco, and many other places. Likewise, the pair channel 6 and 7 was found in Denver and several other places.

With the most financially-affluent and network-connected TV broadcasters all on VHF channels, UHF stations in major population centers of the United States were usually unable to get big TV-network affiliations (ABC, CBS, & NBC), and thus were usually either educational network or independent TV stations.[11] Other UHF stations did for a time affiliate with less-affluent broadcast networks that didn't last very long; for example, the fourth-ranked DuMont Network, which operated from 1946 to 1956, and then went out of business. The movie UHF, which starring "Weird Al" Yankovic and Michael Richards, parodied the independent UHF station phenomenon.[12]

However, there were significant cities that had few or no VHF channels allocated to them. Hence, these cities did get UHF stations that did get major network affiliations and did become financially-sound businesses. Some of these stations have been located in state capital cities or served nearby state capitals, such as Montgomery, Alabama, Frankfort, Kentucky, Dover, Delaware, Lincoln, Nebraska, Topeka, Kansas, Jefferson City, Missouri, Lansing, Michigan, Harrisburg, Pennsylvania, and Madison, Wisconsin. Others were located in cities that were the dominant center for large rural areas—such as Springfield, Missouri, the historic focal point for the Ozarks region. In the United States, television stations of or near state capital cities are important because they closely cover the operations of the state governments and spread the information to the residents of a wide region of the states.

TV antenna manufacturers of years ago often rated their top-of-the-line "deep-fringe" antenna models with phrases like "100 miles VHF / 60 miles UHF" if the antenna included UHF reception at all. (In the practice of electrical engineering, the frequency range in which an antenna is to be used is an important factor in its design.)

TV set manufacturers of years ago often treated UHF tuners as extra-charge optional-items until the All-Channel Receiver Act of 1964 forced their inclusion in all new TV sets as a standard. By 1964, many pioneering UHF broadcasters had already gone bankrupt. Various attempts were made by the FCC regulators to stem the tide of UHF station failures met with mixed results:

The situation was to begin to improve in the 1960s and 1970s, but progress was to be slow and difficult.

While ABC-TV and the short-lived DuMont Network, being smaller and less prosperous networks, had had a number of UHF affiliates,[17] National Educational Television and the later PBS had even more.

The original SIN (Spanish International Network), which was established in 1962 as the predecessor of the modern Univision network, was built primarily by UHF-TV broadcasters, such as charter stations KWEX-TV, Channel 41 in San Antonio and KMEX-TV, Channel 34, in Los Angeles.

Ultimately, in addition to providing TV service where VHF channels simply were not possible because of the limitations on the technology, UHF-TV also became a means to obtain programming which was not being provided by the "Big Three" commercial networks. For example, there were educational services like the Public Broadcasting Service, religious broadcasting, and Spanish language or multilingual broadcasting that all relied primarily on UHF channels to offer programming alternatives.

[edit] Fourth networks, satellite and cable television

In 1970, Ted Turner had acquired a struggling independent station on Channel 17 in Atlanta, Georgia, purchasing reruns of popular television shows, the Atlanta Braves baseball team and the Atlanta Hawks basketball team in order to provide access to entertainment for broadcast.

This station, renamed as WTBS, was uplinked in 1975 to satellite alongside new premium channels such as HBO, gaining access to distant cable television markets and becoming the first of various superstations to obtain national coverage. In 1986 Turner purchased the entire MGM film library, and Turner Broadcasting System's access to movie rights was to prove commercially valuable as home videocassette rental became ubiquitous in the 1980s.

In 1986, the DuMont owned-and-operated station group Metromedia was acquired by News Corporation and used as the foundation to relaunch a fourth commercial network which obtained affiliation with many former big-city independent stations as Fox TV.

While largely built from former independents and UHF stations in its early years, Fox had the large programming budgets that the original DuMont lacked. It ultimately was able in some markets to draw existing long-standing VHF affiliates away from established big-three networks, outbidding CBS for National Football Conference programming in 1994 and attracting many of that network's affiliates. Various smaller networks were created with the intent to follow in its footsteps, often assembling a fledgling network by affiliating with a disparate collection of formerly-independent UHF stations which otherwise would have no network programming.

Fox launched in 1986. The film UHF portrayed a fictional station on channel 62 in 1989. By 1994, New World Communications was moving its established stations from CBS to Fox affiliations in multiple markets, including WJBK-TV 2 Detroit. In many cases, this pushed CBS onto UHF; "U-62" as the new home of CBS in Detroit became CBS owned-and-operated station WWJ-TV in 1995, obtaining access to audiences thousands of miles distant through satellite and cable television.

The concentration of media ownership, the proliferation of cable and satellite television and the digital television transition have contributed to the quality equalization of VHF and UHF broadcasts. The distinction between UHF and VHF characteristics has declined in importance with the emergence of additional broadcast television networks (Fox, The CW, MyNetworkTV, Univision, Telemundo and ION), and the decline of direct OTA reception. The number of major large-city independent stations has also declined as many have joined or formed new networks.

[edit] Digital television

See also DTV transition in the United States#VHF_frequencies_and_digital_television

The majority of digital TV stations currently broadcast their over-the-air signals in the UHF band, both because VHF had been largely already filled with analog TV at the time the digital facilities were built and because of severe issues with impulse noise on digital low-VHF channels. While virtual channel numbering schemes routinely display channel numbers like "2.1" or "6.1" for individual North American terrestrial HDTV broadcasts, these are more often than not actually UHF signals. Many equipment vendors therefore use "HDTV antenna" or similar branding as all but synonymous to "UHF antenna".

Terrestrial digital television is based on a forward error correction scheme, in which a channel is assumed to have a random bit error rate and additional data bits may be sent to allow these errors to be corrected at the receiver. While this error correction can work well in the UHF band where the interference consist largely of white noise, it has largely proven inadequate on lower VHF channels where bursts of impulse noise disrupt the entire channel for short lengths of time. A short impulse-noise burst might be a minor annoyance to analog TV viewers, but due to the fixed timing and repetitive nature of analog video synchronization is usually recoverable. The same interference can prove severe enough to prevent the reliable reception of the more fragile and more highly-compressed ATSC digital television. Power limits are also lower on low-VHF; a digital UHF station may be licensed to transmit up to a megawatt of effective radiated power. Very few stations returned to VHF channels 2-6 after digital transition was completed in 2009. At least three quarters of all full-power digital broadcasts continued to use UHF transmitters, even after transition is complete, with most of the others located on the high-VHF channels. In some American markets, such as Syracuse, New York, there are no full-service VHF TV stations remaining after digital transition.

The one remaining limitation of UHF, that of a greatly-reduced ability for signals to travel great distances in the presence of obstacles due to terrain, continues to adversely affect digital UHF TV reception. Potentially, this limitation could be overcome by the use of DTS (Distributed Transmission Systems). Multiple digital UHF transmitters in carefully-selected locations can be synchronized as a single-frequency network to produce a tailored coverage area pattern rivaling that of a single full-power VHF transmitter.

While the Federal Communications Commission authorization to use DTS on anything more than an experimental basis came in November 2008, too late for sites to be acquired and transmitters built before the 2009 end of American digital transition, it is likely that more of these distributed UHF transmission systems will be constructed alongside conventional digital broadcast translator systems in the years to come as a means to get digital and high-definition television out to a wider audience.

[edit] UHF islands

One notable exception to historical patterns favoring VHF broadcasters has existed in mid-sized television markets within the United States which were too close to the outer fringe of the broadcast range of large-city VHF stations to qualify for their own stations on these frequencies. As no full-power VHF channels could be made available in these areas without encountering problems of interference from overlapping broadcast ranges, the Federal Communications Commission granted some mid-sized cities only UHF licenses. With all stations (including big-three network affiliates) on UHF, all-channel receivers and antennas became commonplace locally and UHF stations signing on as early as 1954 were often able to obtain the programming and viewership needed to remain viable into the modern era.[18]

These communities, known as UHF islands, included cities like Huntsville, Alabama; Fresno, California; South Bend, Indiana; Elmira, New York; Scranton, Pennsylvania; Fort Wayne, Indiana; Lexington, Kentucky, and Springfield, Massachusetts. Other smaller cities found that only one VHF channel was open and any additional programming would need to be provided either by UHF, by distant stations or by low-power broadcasting.

[edit] Broadcast translators and low-power television

A large number of very small UHF TV transmitters continue to operate with no programming or commercial identity of their own, merely retransmitting signals of existing full-power stations to a smaller area poorly covered by the main VHF signal. Such transmitters are called "translators" rather than “stations”. The smallest, owned by local municipal-level groups or the originating TV stations, are numbered sequentially - W or K, followed by the channel number, followed by two sequentially-issued letters, yielding a "translator callsign" in a generic format which appears K14AA through W69ZZ. Translators and repeaters also exist on VHF channels, but infrequently and with stringently-limited power as the VHF spectrum is already crowded with full-power network stations in most regions.

The translator band, UHF TV channels 70-83, consisted mostly of these small repeaters; it was removed from television use in 1983 with the tiny repeaters moved primarily to lower UHF channels. The 804-890 MHz band segment is now primarily used by mobile phones.

As improvements to originating stations signals lessen the need for these small translators in some areas, often the small transmitter facilities and their allocated frequencies would be repurposed for low-power broadcasting; instead of repeating a distant signal, the tiny transmitter would be used to originate programming for a small local area.

[edit] Radio, mobile and non-broadcast applications

The Family Radio Service and General Mobile Radio Service use the 462 and 467 MHz areas of the UHF spectrum. There is a considerable amount of lawful unlicensed activity (cordless phones, wireless networking) clustered around 900 MHz and 2.4 GHz. These ISM bands - open frequencies with a higher unlicensed power permitted for use originally by Industrial, Scientific, Medical apparatus - are now becoming some of the most crowded in the spectrum because they are open to everyone.

The 2.45 GHz frequency, readily absorbed by water, is the standard for use by microwave ovens.

The spectrum from 806 MHz to 890 MHz (UHF channels 70-83) was taken away from TV broadcast services in 1983, primarily for analogue mobile telephony. In 2009, as part of the transition from analog to digital over-the-air broadcast of television, the spectrum from 698 MHz to 806 MHz (UHF channels 52-69) was also no longer used for TV broadcasting. Channel 55, for instance, was sold to Qualcomm for their MediaFLO service, which is resold under various mobile telephone network brands. Some US broadcasters had been offered incentives to vacate this channel early, permitting its immediate mobile use.

The FCC's scheduled auction for this newly-available spectrum was completed in March 2008.[19]

[edit] Frequency allocation

[edit] Australia

[edit] Canada

[edit] United Kingdom

[edit] United States

A brief summary of some UHF frequency use:

[edit] See also

[edit] References

  1. ^ http://www.ic.gc.ca/eic/site/smt-gst.nsf/vwapj/DTV_PLAN_Dec08-e.pdf/$file/DTV_PLAN_Dec08-e.pdf
  2. ^ About DAB - Canadian Association of Broadcasters
  3. ^ Fifties Television: The Industry and Its Critics, William Boddy, University of Illinois Press, 1992, ISBN 9780252062995
  4. ^ http://www.nrcdxas.org/articles/1945ass.txt
  5. ^ Radio and Television Regulation: Broadcast Technology in the United States 1920-1960, Hugh Richard Slotten, JHU Press, 2000, ISBN 9780801864506
  6. ^ Missed Opportunities: FCC Commissioner Frieda Hennock and the UHF Debacle, Susan L. Brinson, Journal of Broadcasting & Electronic Media • Spring, 2000
  7. ^ VALVES AT UHF: A REVIEW OF RECENT DEVELOPMENTS, S. Simpson, Practical Television magazine, March 1954
  8. ^ TV-technology.com The Superheterodyne Concept and Reception, Charles W. Rhodes, TV Technology, July 20, 2005
  9. ^ http://tulsatvmemories.com/tvthesi3.html
  10. ^ Stay Tuned: A History of American Broadcasting; pp 387-388; Christopher H. Sterling, John M. Kittross; Erlbaum 2002; ISBN 9780805826241
  11. ^ UHF morgue
  12. ^ U-62 program schedule, July 1989
  13. ^ Buffalo Broadcasters: History - UHF
  14. ^ Media Economics: Theory and Practice, Alison Alexander, Erlbaum Associates 2004 ISBN 9780805845808
  15. ^ FCC order revoking secondary affiliation rule, 1995
  16. ^ http://www.wcl.american.edu/journal/lawrev/53/rothenberger.pdf
  17. ^ The DuMont Television Network, Appendix 10/11: A Trail of Bleached Bones, C. Ingram
  18. ^ WSJV 28 South Bend, Indiana history indicates station founded 1954, still extant as no VHF channels available due to proximity to Chicago
  19. ^ Going once, twice, the 700MHz spectrum is sold, NY Times, Mar 18 2008
  20. ^ http://www.broadcasting-history.ca/stations/radio/Digital_Audio_Broadcasting.html
  21. ^ http://www.raytheon.com/capabilities/rtnwcm/groups/ncs/documents/content/rtn_ncs_products_arc164_pdf.pdf

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